Yesterday, Trolltech released the final version of Qt 4.4, their graphical toolkit which forms the base for, among a lot of other things, the KDE project. It still features the dual-license model (of course), so proprietary developers can license Qt, while open source developers can get a GPLd version (both GPL 2 as well as 3). Read on for a quick overview of the new features, as well as some findings by Ars Technica.

There are some other good GUI toolkits with licenses that allow both use in free software and proprietary software. So, this can be an advantage of some toolkits other than Qt.

It's more complicated than that. At face value, it seems like a disadvantage that Qt costs money for commercial development. However, this price gives you commercial support and some promises of future viability. This is an investment in your tool and your tool makes your work easier and more efficient - so this will give some returns. If you were to pay $0 for a toolkit which would stop being developed or advancing at a reasonable rate after your project is far along, dealing with the situation would probably be even more expensive.